Pharmaceutical preparations, food supplements or other powdery or granular substances are frequently provided in precisely measured consumer units. To this end, target containers such as hard gelatin capsules, blister packs or the like are filled with correspondingly measured quantities of powder. As a rule, in this case, volumetric metering of the individual portions is performed, the desired target weight being produced from the metering volume and a density distribution which is as reproducible as possible. Along with panel, roller or membrane metering, it is also possible to use so-called pipettes for such volumetric metering. Such pipettes include a pipette sleeve and a metering piston. On its inside, the pipette sleeve includes an inner circumferential wall which is closed about its longitudinal axis. The inner circumferential wall extends in the direction of the longitudinal axis up to the free end of the pipette sleeve and ends there in a pipette edge which extends around the longitudinal edge. The metering piston is guided displaceably parallel to the longitudinal axis inside the inner circumferential wall. In this case, the end face of the metering piston, in the retracted metering position thereof, defines a metering chamber together with the inner circumferential wall.
A powder bed is prepared for the metering operation. One or multiple pipettes are plunged into the powder bed from above. The respective metering piston is retracted relative to the pipette edge by such an amount that, in the metering position then assumed, it provides the desired volume of the metering chamber. During the plunging operation, the metering chamber is filled with the powder thereby realizing a powder pellet with the desired target volume. On account of the inherent static friction forces present, the powder pellet initially remains in the metering chamber and can be lifted out of the powder bed together with the pipette. The respective pipette is then positioned above an assigned target container. The powder pellet is then ejected via the metering piston and falls into the target container intended for it.
Observations have shown that the ejection operation of the powder pellet is not without problems. In particular, it could be observed that the powder pellet, initially positioned upright in the pipette, does not remain in the upright position when being ejected and falling down, but tends to tilt sideways. Such sideways tilting impairs the target accuracy of the falling powder pellet. It has to be ensured in an expensive and time-consuming manner that the powder pellet passes entirely into the target container without colliding with the edges of the container on account of its tilting movement. In particular in the case of target containers with small fill openings compared to the size of the powder pellet, as is the case with two-piece capsules, this can result in powder losses and consequently in a reduced yield of good capsules.
Furthermore, the use of pipettes in conjunction with 100% in-process control has proved to be difficult. The use of AMV sensors (Advanced Mass Verification) for capacitive measuring of the metered powder units has up to now only supplied insufficiently precise results.